An OpEntryPoint with the PostDepthCoverage Execution Model must also set
the EarlyFragmentTests Execution Mode.

Issues

What should we call the capability?

DISCUSSION: Can both the execution mode and capability have the same name?
It seems like it could be confusing (documentation and code-wise) even if it is
technically possible. The Capability suffix would be redundant.

RESOLVED: Call it SampleMaskPostDepthCoverage, similar to
SampleMaskCoverageOverrideNV since this is modifying the semantics of
the SampleMask decorated variables. Other options considered were
PostDepthCoverage or PostDepthCoverageCapability.

Should the EarlyFragmentTests Execution Mode be explicit or implicit when
PostDepthCoverage is enabled?

RESOLVED: In GL_EXT_post_depth_coverage, both the early_fragment_test and
post_depth_coverage layouts needed to be explicitly set. In
GL_ARB_post_depth_coverage, the early_fragment_test was made implicit when
post_depth_coverage was enabled as there is no other sensible way of using
post depth coverage. However, since SPIR-V is lower-level than GLSL and more
explicit/verbose, it follows that both Execution Modes should be explicitly
declared and the GLSL front-end can ensure that both modes are specified when
either extension is used, and it should also be simpler for consumers.